This invention is generally directed to amorphous silicon imaging members; and more specifically, the present invention is directed to layered photoresponsive imaging members, or devices comprised of hydrogenated amorphous silicon and overcoating layers of certain nonstoichiometric silicon itrides. In one embodiment of the present invention, there is provided a layered photoresponsive imaging member comprised of a supporting substrate, a blocking layer of hydrogenated amorphous silicon with dopants therein, a bulk photoconducting layer of hydrogenated amorphous silicon with dopants therein, and in contact therewith an overcoating layer of silicon nitride containing in a specific amount an excess of silicon. Further, in another specific embodiment of the present invention there is provided a layered photoresponsive imaging member comprised of a supporting substrate, a blocking layer of hydrogenated amorphous silicon with high, for example about 100 parts per million concentration of boron therein, a bulk photoconducting layer of hydrogenated amorphous silicon with minor amounts of boron therein, for example 3 parts per million; and in contact therewith an overcoating layer of silicon nitride with an excess of silicon. These imaging members can be incorporated into electrophotographic; and in particular xerographic imaging and printing systems wherein, for example, the latent electrostatic patterns which are formed can be developed into images of high quality and excellent resolution. Moreover, the members of the present invention possess high charge acceptance values, in excess of 40 volts/micron for example; and further these members can be of a very desirable thickness from, for example about 100 microns or less. Also, the imaging members of the present invention have desirable low dark decay properties enabling them to be very useful in xerographic imaging processes. In these processes, latent electrostatic images are formed on the devices involved followed by development, transfer and fixing. Additionally, the photoresponsive imaging members of the present invention when incorporated into xerographic imaging and printing systems are insensitive to humidity and corona ions generated permitting the formation of acceptable images of high resolution for an extended number of imaging cycles. Also, the imaging members of the present invention enable the elimination of undesirable lateral movement of charges at the interface between the photoconducting layer and the overcoating layer as a result of band bending, a prior art problem, and thus permitting images with increased resolution and less print deletions.
Electrostatographic imaging, particularly xerographic imaging processes, are well known, and are extensively described in the prior art. In these processes a photoresponsive or photoconductor material is selected for forming the latent electrostatic image thereon. The photoreceptor is generally comprised of a conductive substrate containing on its surface a layer of photoconductive material; and in many instances, a thin barrier layer is situated therebetween to prevent charge injection from the substrate, which could adversely affect the quality of the resulting image. Examples of known useful photoconductive materials include amorphous selenium, alloys of selenium such as selenium-tellurium, selenium-arsenic, and the like. Additionally, there can be selected as the photoresponsive imaging member various organic photoconductive materials including, for example, complexes of trinitrofluorenone and polyvinylcarbazole. Recently there has been disclosed layered organic photoresponsive devices with aryl amine hole transporting molecules, and photogenerating layers, reference U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference.
Also known are amorphous silicon photoconductors, reference for example U.S. Pat. Nos. 4,265,991 and 4,225,222. There is disclosed in the '991 patent an electrophotographic photosensitive member comprised of a substrate, and a photoconductive overlayer of amorphous silicon containing 10 to 40 atomic percent of hydrogen and having a thickness of 5 to 80 microns. Additionally, this patent describes several processes for preparing amorphous silicon. In one process embodiment, there is prepared an electrophotographic photosensitive member by heating the member present in a chamber to a temperature of 50.degree. C. to 350.degree. C., introducing a gas with silicon and hydrogen atoms, providing an electrical discharge in the chamber by electric energy to ionize the gas, followed by depositing amorphous silicon on an electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second by utilizing an electric discharge thereby resulting in an amorphous silicon photoconductive layer of a predetermined thickness. Although the amorphous silicon device described in this patent is photosensitive, after a minimum number of imaging cycles, less than about 1,000 for example, unacceptable low quality images of poor resolution with many deletions may result. With further cycling, that is subsequent to 1,000 imaging cycles and after 10,000 imaging cycles, the image quality may continue to deteriorate often until images are partially deleted.
Further, there is disclosed in the prior art amorphous silicon photoreceptor imaging members containing, for example, stoichiometric silicon nitride overcoatings; however, these members in some instances generate prints of low resolution as a result of the band bending phenomena. Additionally, with the aforementioned silicon nitride overcoatings, the resolution loss can in many instances be extreme thereby preventing, for example, any image formation whatsoever.
There are also illustrated in copending applications photoconductive imaging members comprised of amorphous silicon. Accordingly, for example, there is illustrated in copending application U.S. Ser. No. 695,990, entitled Electrophotographic Devices Containing Compensated Amorphous Silicon Compositions, the disclosure of which is totally incorporated herein by reference, an imaging member comprised of a supporting substrate and an amorphous hydrogenated silicon composition containing from about 25 parts per million by weight to about 1 percent by weight of boron compensated with substantially equal amounts of phosphorous. Furthermore, described in copending application U.S. Pat. No. 4,544,617 entitled Electrophotographic Devices Containing Overcoated Amorphous Silicon Compositions, the disclosure of which is totally incorporated herein by reference, are imaging members comprised of a supporting substrate, an amorphous silicon layer, a trapping layer comprised of doped amorphous silicon, and a top overcoating layer of stoichiometric silicon nitrides. More specifically, there is disclosed in this copending application an imaging member comprised of a supporting substrate, a carrier transport layer comprised of uncompensated or undoped amorphous silicon; or amorphous silicon slightly doped with p or n type dopants such as boron or phosphorous, a thin trapping layer comprised of amorphous silicon which is heavily doped with p or n type dopants such as boron or phosphorous; and a top overcoating layer of specific stoichiometric silicon nitride, silicon carbide, or amorphous carbon. However, one disadvantage with this imaging member is that the trapping layer introduces a dark decay component which reduces the charge acceptance for the imaging member.
Additionally, described in copending application U.S. Pat. No. 4,613,556 entitled Heterogeneous Electrophotographic Imaging Members of Amorphous Silicon, the disclosure of which is totally incorporated herein by reference, are imaging members comprised of hydrogenated amorphous silicon photogenerating compositions, and a charge transporting layer of plasma deposited silicon oxide.
Other representative prior art disclosing amorphous silicon imaging members, including those with overcoatings, are U.S. Pat. Nos. 4,460,669; 4,465,750; 4,394,426; 4,394,425; 4,409,308; 4,414,319; 4,443,529; 4,452,874; 4,452,875; 4,483,911; 4,359,512; 4,403,026; 4,416,962; 4,423,133; 4,460,670; 4,461,820; 4,484,809; and 4,490,453. Additionally, patents that may be of background interest with respect to amorphous silicon photoreceptor members include, for example, 4,359,512; 4,377,628; 4,420,546; 4,471,042; 4,477,549; 4,486,521; and 4,490,454.
Further, additional representative prior art patents that disclose amorphous silicon imaging members include, for example, U.S. Pat. No. 4,357,179 directed to methods for preparing imaging members containing high density amorphous silicon or germanium; U.S. Pat. No. 4,237,501 which discloses a method for preparing hydrogenated amorphous silicon wherein ammonia is introduced into a reaction chamber; U.S. Pat. Nos. 4,359,514; 4,404,076; 4,403,026; 4,397,933; 4,423,133; 4,461,819, 4,237,151; 4,356,246; 4,361,638; 4,365,013; 3,160,521; 3,160,522; 3,496,037; 4,394,426; and 3,892,650. Of specific interest are the amorphous silicon photoreceptors illustrated in U.S. Pat. Nos. 4,394,425; 4,394,426 and 4,409,308 wherein overcoatings such as silicon nitride and silicon carbide are selected. Examples of silicon nitride overcoatings include those with a nitrogen content of from about 43 to about 60 atomic percent.
Additionally, processes for depositing large area defect free films of amorphous silicon by the glow discharge of silane gases are described in Chittick et al., the Journal of the Electrochemical Society, Volume 116, Page 77, (1969). Further, the fabrication and optimization of substrate temperatures during amorphous silicon fabrication is illustrated by Walter Spear, the Fifth International Conference on Amorphous and Liquid Semiconductors presented at Garmisch Partenkirchen, West Germany in 1963. Other silicon fabrication processes are described in the Journal of Noncrystalline Solids, Volumes 8 to 10, Page 727, (1972), and the Journal of Noncrystalline Solids, Volume 13, Page 55, (1973).
Although the above described amorphous silicon photoresponsive members, particularly those disclosed in the copending applications, are suitable, in most instances, for their intended purposes there continues to be a need for improved members comprised of amorphous silicon which can be easily fabricated. Additionally, there is a need for amorphous silicon imaging members that possess desirable high charge acceptance values and low charge loss characteristics in the dark. Furthermore, there continues to be a need for improved amorphous silicon imaging members with overcoating layers of specific nonstoichiometric silicon nitrides enabling the substantial elimination of the undesirable lateral motion of charge, and thereby allowing for the generation of images of increased resolution when compared to amorphous silicon imaging members with other overcoatings of silicon nitride. Additionally, there is a need for improved layered imaging members of amorphous silicon which are humidity insensitive and are not adversely effected by electrical consequences resulting from scratching and abrasion. There is also a need for amorphous silicon imaging members which can be selected for use in repetitive imaging and printing systems. Furthermore, there is a need for amorphous silicon imaging members with low surface potential decay rates in the dark, and photosensitivity in the visible and near visible wavelength range. Further, there is a need for improved layered amorphous silicon imaging members which have very few image defects such as white spots with images of dark solids.